One of the important themes in calculus is the analysis of relationship between physical or mathematical quantities. Such relationships can be describes in term of graphs, formulas, numerical data, or words. In this chapter we will develop the concept of a function, which is the basic idea that underlies almost all mathematical and physical relationships, regardless of the form in which they are expressed. For readers who would like to pursue applications in more depth, we will provide an optional section that introduces the use of regression curve to model real-world data. We will conclude the chapter with a discussion of curves in the plane that are most conveniently described using a pair of functions. (This material on “parametric curves” may by postponed until later if desired.)